Electrochemical cell comprising gamma MnO2 cathode having filamentary protrusions

ABSTRACT

The invention relates to the manufacture of manganese dioxide by a chemical process. The resulting manganese dioxide product takes the form of particles characterized by filament-like protrusions jutting out from its surface. The manganese dioxide particles having such surface features can be manufactured by reacting manganese sulfate with sodium peroxodisulfate in an aqueous solution. The process can be controlled to yield high density manganese dioxide. The manganese dioxide formed in the process can be deposited directly onto the surface of electrolytic manganese dioxide (EMD). The manganese dioxide product is particularly suitable for use as a cathode active material in electrochemical cells.

This application is a division of application Ser. No. 08/122,966, filedSep. 20, 1993, now U.S. Pat. No. 5,348,726 which is a division of U.S.Ser. No. 07/952,034, filed Sep. 28, 1992, now U.S. Pat. No. 5,277,890.

The invention relates to a process for production of manganese dioxide,particularly for use as a cathode active material in electrochemicalcells.

Manganese dioxide is commonly employed as a cathode active material incommercial batteries including heavy duty, alkaline and lithium cells.Battery grade manganese dioxide has been derived from naturallyoccurring manganese dioxide (NMD) and synthetically produced manganesedioxide. Synthetic manganese dioxide is basically divided into twocategories: electrolytic manganese dioxide (EMD) and chemical manganesedioxide (CMD). NMD because of its high impurity content cannot beemployed in alkaline or lithium cells.

EMD, which is typically manufactured from direct electrolysis of a bathof manganese sulfate and sulfuric acid, is a high purity, high density,gamma manganese dioxide which has been proved to be desirable for use asa cathode active material in alkaline and lithium cells. During theelectrolysis process the gamma EMD is deposited directly on the anodewhich is typically made of titanium, lead alloy or carbon. The thicknessof the MnO₂ deposited on the anode depends on the current density andelectrolysis time. The EMD deposit is removed from the anode, crushed,ground, washed, neutralized and dried in a rotary dryer. The EMD productis generally heat treated before use in a lithium dry cell. Processesfor the manufacture of EMD and its properties appear in Batteries,edited by Karl V. Kordesch, Marcel Dekker, Inc., New York, Vol. 1,(1974), p. 433-488.

CMD has for many years been economically produced commercially, but suchcommercial chemical processes while yielding high purity MnO₂, do notyield densities of MnO₂ comparable to that of EMD. As a result EMD hasbecome the most widely used form of battery grade MnO₂, particularly foralkaline and lithium cells, since in such application it has become mostdesirable to employ high density MnO₂ to increase the capacity of thesecells. However, in the course of the manufacture of EMD, it is difficultto significantly alter important properties, such as surface area andactivity, without adversely affecting the density. Also the compact,smooth particle structure of EMD and its low surface area can be adisadvantage, particularly, in application to lithium cells.

U.S. Pat. No. 2,956,860 (Welsh) discloses a chemical process for themanufacture of battery grade MnO₂ by employing the reaction mixture ofMnSO₄ and an alkali metal chlorate, preferably NaClO₃. This process isknown in the art as the "Sedema process" for manufacture of chemicalmanganese dioxide (CMD). The reaction is carried out in the presence ofsolid MnO₂ particles which act as a catalyst and nucleation site fordeposition of the MnO₂ formed from the reaction of MnSO₄ and alkalimetal chlorate. As the reaction proceeds, MnO₂ which is formedprecipitates onto, and even into, the MnO₂ substrate particles. Theresulting MnO₂ product from the Sedema process takes the form ofsmooth-surfaced spherical particles. However, the MnO₂ does not have adensity as high as that obtained in EMD. Significantly higher densitiesof the MnO₂ product are not obtainable by controlling reaction rate withalkali metal chlorate. Also the MnO₂ produced from the process disclosedin this reference cannot be readily deposited on subtrates other thanmanganese oxides. If an alternative substrate or no substrate isemployed, the MnO₂ product precipitates out during formation as a lightfluffy product which is unacceptable as battery grade MnO₂.

An article by P. Strobel and J. C. Charenton (Revue de Chimie Minerale,Vol. 23 (1986), p.125-137, at p. 130) discloses a reaction of potassiumor ammonium peroxodisulfates (K₂ S₂ O₈ or (NH₄)₂ S₂ O₈) with MnSO₄ toyield mixtures of hollandite (alpha MnO₂) and gamma MnO₂. The referencestates, at p. 129, that only potassium and ammonium peroxodisulfates aresufficiently stable for use in the reaction.

An article by K. Yamamura et. al., ("A New Chemical Manganese Dioxidefor Dry Batteries," Progress in Batteries & Battery Materials, Vol. 10(1991), p. 56-75) discloses another process for manufacturing gammaMnO₂. The process referenced as the "CELLMAX" (CMD-U) process involvesspecial treatment of purified crystalline MnSO₄ to produce anelectrochemically active high density gamma MnO₂. The product has asurface area and particle appearance similar to electrolytic manganesedioxide (EMD), but differs in its pore size, tap density and particlesize distribution. The process consists of the steps of leachingmanganese ore, crystallizing, adjusting the pH, compressing andgrinding. In the process the manganese sulfate solution extracted fromthe manganese ore is purified, crystallized under optimum conditions androasted at very high temperature. The product Mn₃ O₄ is oxidized to Mn₂O₃ by oxygen at high temperature. The Mn₂ O₃ is subjected to acidtreatment to yield gamma MnO₂ which in turn is compressed to yield ahigher density. Although a high density gamma MnO.sub. 2 product isreported, the process has the disadvantage of involving a number ofreaction and processing steps which require careful control and would beexpensive to implement.

It would be desirable to have a practical chemical process forproduction of high purity battery grade manganese dioxide (CMD) whichhas a density comparable to electrolytic manganese dioxide (EMD).

It would be desirable to be able to have greater control over otherphysical characteristics, such as surface area, activity and porosity ofthe CMD without significantly sacrificing density.

It would be desirable to improve the performance and capacity ofalkaline and lithium cells over that currently obtainable with EMDcathode active material.

The features of the product of the invention will be better appreciatedwith reference to the following figures:

FIG. 1A is an electron photomicrograph showing the MnO₂ particles fromthe process of the invention carried out at slow rate of heating ofreactants.

FIG. 1B is an electron photomicrograph of the particles in FIG. 1Aenlarged to show the filament-like surface protrusions.

FIG. 2A is an electron photomicrograph showing smaller sized MnO₂particles from the process of the invention carried out at fast rate ofheating of the reactants.

FIG. 2B is an electron photomicrograph of the particles in FIG. 2Aenlarged 9850× to show the filament-like surface protrusions.

FIG. 3A is an electron photomicrograph of EMD particles (prior art).

FIG. 3B is an electron photomicrograph of the EMD particles in FIG. 3Aenlarged to show the characteristically irregular particle shape andsmooth surface structure.

FIG. 4A is en electron photomicrograph of EMD particles coated with MnO₂produced by the process of the invention.

FIG. 4B is an electron photomicrograph of coated EMD particles in FIG.4A enlarged to show the filament-like surface protrusions.

FIG. 5A is an electron photomicrograph of prior art chemical manganesedioxide (CMD) particles.

FIG. 5B is an electron photomicrograph of the particles in FIG. 5Aenlarged to show surface features.

FIG. 6A is a graphical plot of the voltage profile (voltage versusservice hours) in an alkaline AA cell at 3.9 ohm constant load,comparing performance of the MnO₂ of the invention (P-CMD) withconventional EMD.

FIG. 6B is a graphical plot of the voltage profile (voltage versus milliamp-hour per gram MnO₂) in a flooded alkaline cell at 0.3 milli-amp/cm²current drain rate, comparing performance of the MnO₂ product of theinvention (P-CMD) with conventional EMD.

FIG. 7A is a graphical plot of the voltage profile (voltage versus milliamp-hour per gram MnO₂) in a lithium cell at 0.17 milli-amp/cm² currentdrain rate comparing performance of the MnO₂ product of the invention(P-CMD) with conventional EMD.

FIG. 7B is a graphical plot of the voltage profile (voltage versus milliamp-hour per gram MnO₂) in a lithium cell at 1.0 milli-amp/cm² currentdrain rate, comparing performance of the MnO₂ product of the invention(P-CMD) with conventional EMD.

FIG. 8A is a graphical plot of the voltage profile (voltage versus milliamp-hour per gram MnO₂) in a lithium cell at 0.17 milli-amp/cm² currentdrain rate, comparing performance of the MnO₂ product of the invention(P-CMD) with conventional CMD (WSLi).

FIG. 8B is a graphical plot of the voltage profile (voltage versus milliamp-hour per gram MnO₂) in a lithium cell at 1.0 milli-amp/cm² currentdrain rate, comparing performance of the MnO₂ product of the invention(P-CMD) with conventional CMD (WSLi).

The present invention involves a process for production of battery gradechemical manganese dioxide (CMD). The CMD product of the invention whenused in electrochemical cells, particularly alkaline and lithium cells,provides these cells with higher capacity and energy density per gramthan are obtainable from the same cells employing electrolytic manganesedioxide (EMD). The process of the invention allows for greater controlof properties such as density, surface area and particle size than ispossible with present processes for the manufacture of conventionalforms of EMD or CMD. The process of the invention therefore allows forproduction of high purity CMD which can be made to have properties morenearly optimal for a given electrochemical cell or battery type. Inparticular a high density of the MnO₂ product is obtainable by theprocess of the invention. The high density of the MnO₂ product iscomparable to that obtained from electrolytic manganese dioxide (EMD),yet the surface area of each MnO₂ particle is greater than that obtainedfrom conventional EMD and CMD processes. The high useful surface area ofeach particle allows for better performance, particularly in lithiumcells containing MnO₂. By "useful" surface area we refer to the surfacearea which is accessible to the electrolyte.

The process of the invention for production of battery grade manganesedioxide is carried out principally by reacting an aqueous solution ofmanganese sulfate with sodium peroxodisulfate.

The reaction may be represented as follows:

    MnSO.sub.4 +Na.sub.2 S.sub.2 O.sub.8 +2H.sub.2 O=MnO.sub.2 +Na.sub.2 SO.sub.4 +2H.sub.2 SO.sub.4                               (I)

When an aqueous solution of manganese sulfate (MnSO₄) is reacted withsodium peroxodisulfate (Na₂ S₂ O₈), a gamma crystal structure of MnO₂ isdirectly obtainable as a reaction product in the form of a precipitate.The MnO₂ precipitate tends to form spherical particles havingfilament-like protrusions emanating from each particle surface. Thefilament-like protrusions are crystalline and appear as filaments,hairs, fibers or needles which radiate outwardly (typically straightout) from the surface of each MnO₂ particle and are uniformlydistributed over the particle surface. The term "filament-like" as usedherein shall be construed as including thin, elongated, protrudingstructures such as but not limited to filaments, hairs, needles andfibrous structures. They have an elongated backbone or spine structurealong a major portion of their length and appear to be uniformly anddensely distributed over the particle surface. The "filament-like"protrusions are characterized by a length to width ratio between about2:1 and 20:1, typically between about 3:1 and 10:1, wherein the widthand length refer to those portions of the protrusions which are visiblefrom the particle surface. The average length of the "filament-like"protrusions is typically between 0.3 to 1 micron and the average widthis typically between 0.1 to 0.3 micron. These dimensions are measurableat a magnification of about 40,000 times actual size. The"filament-like" protrusions result in high surface area of the MnO₂particle. The MnO₂ particles of the invention as above described maytypically be referenced as P-CMD (a new name) in several of the figures.

Unlike the well known Sedema process as disclosed in U.S. Pat. No.2,956,860 (above referenced), the present invention permits the averageparticle size and density of the MnO₂ product to be altered byregulating the rate of the above reaction (I). This can be accomplishedby simply controlling the amount or rate of heat supplied to thereaction. Unlike the Sedema process the present reaction does notrequire a catalytic MnO₂ substrate for receiving the MnO₂ product. Infact no catalyst is required and the MnO₂ product forms into dense,discrete particles without the need of a substrate material. However, ithas been discovered that the reaction mixture can be seeded with almostany nonreactive solid material including metals and such material willact as a substrate for the MnO₂ product. That is, the MnO₂ reactionproduct will precipitate directly on the solid material.

It has been discovered that the above reaction mixture can be seededwith particles of electrolytic manganese dioxide (EMD) and the MnO₂reaction product will deposit directly on the EMD. This results in avery high density hybrid gamma MnO₂ whose outer surface comprises anMnO₂ coating having filament-like protrusions and high surface area,while the overall particle shape and interior structure is thatcharacteristic of EMD. This hybrid form of MnO₂ may be used as cathodeactive material in conventional electrochemical cells, particularlyalkaline or lithium cells. It is especially attractive for use inlithium cells, since the exposure of the EMD particles to H₂ SO₄ duringthe reaction of the invention, leaches out small amounts of sodium thatis trapped within the EMD particles. This reduces the amount of sodiumimpurity in the MnO₂ product, which is particularly advantageous if itis to be used as cathode active material in lithium cells. It has alsobeen discovered that the reaction mixture can be advantageously seededwith graphite or carbon black particles. In such case the MnO₂ reactionproduct will deposit directly onto the surface of these particles toform a hybrid particulate material which may be used as cathode activematerial in conventional electrochemical cells, particularly alkaline orlithium cells.

The above reaction (I) may typically be carried out in a temperaturerange between about 30° and 100° C., preferably between 70° and 90° C.The reaction (I) is preferably carried out in a temperature rangebetween about 70° C. and 80° C. when the intended use of the MnO₂product is as a cathode active material in an alkaline cell, and betweenabout 80° and 90° C. when the intended use is as a cathode activematerial in a lithium cell. (For end application of the MnO₂ product toalkaline cells it is preferable to keep the final temperature below 85°C. in order to obtain a gamma MnO₂ product with higher running voltageand capacity than EMD.) After the reaction is complete, the MnO₂precipitate is collected and rinsed with distilled water until it has apH of 7. It may then be dried at room temperature if its intended use isas a cathode active material in an alkaline cell. Alternatively, it maybe dried at elevated temperature for more thorough drying, if itsintended use is as cathode active material in a lithium cell. Theresulting dry gamma MnO₂ has a high purity and low sodium content ofless than about 500 ppm. The dry MnO₂ product contains at least 95%gamma MnO₂ in particulate form. (No other crystalline forms of MnO₂ havebeen detected in the dry MnO₂ product of the invention, but 95% is thelimit of resolution of the x-ray diffraction analysis employed forMnO₂.) Every MnO₂ particle made by the process of the invention, whenobserved at a magnification between 200 and 9850 times actual size,typically between 200 and 2000 times actual size, appears to havefilament-like protrusions radiating outwardly from the particle surfaceand these protrusions appear to be uniformly distributed around theparticle surface. The gamma MnO₂ so produced may then be heat treated inconventional manner to convert it to a gamma-beta variety, if desired.This treatment is preferred if the end use of the MnO₂ is as cathodeactive material in lithium cells. The heat treatment is well known, asuitable heat treatment process being disclosed in U.S. Pat. No.4,921,689.

The gamma MnO₂ product of the invention can be compacted and used ascathode active material in conventional Zn/MnO₂ alkaline cells orLi/MnO₂ lithium cells. It results in a cell having increased capacityand energy density per gram than obtained with EMD cathode activematerial for the same cell. The gamma MnO₂ can also be used as acatalyst in zinc/air cells.

It has been determined that various properties of the MnO₂ product canbe altered and controlled by controlling the rate at which the reactionmixture is heated. In general a denser MnO₂ product is obtained if thereaction is carried out at a slower rate, e.g., if heat is supplied tothe reaction at a slower rate. In a slower reaction individual particlesof MnO₂ have time to grow to form larger more compact particles. In afaster reaction, e.g. produced by faster heating of the reactionmixture, the individual particles of the MnO₂ product do not havesufficient time to grow to form larger particles. Therefore theindividual particles are smaller and less compact. They have a fluffierappearance and lower average density than particles obtained from aslower reaction.

A sufficiently slow reaction rate to provide an MnO₂ product bulkdensity of about 15 to 32 g/in³ (0.9 and 2 g/cm³) SAD (Scott ApparentDensity) is obtained if the aqueous reaction mixture of MnSO₄ and Na₂ S₂O₈ is maintained at an initial temperature of about 50° C. for about 18hours and then slowly increased at nearly constant rate for betweenabout 5 and 10 hours until a final temperature of between about 70° to90° C. is obtained. The reaction mix may then be left to stand for about1 hour at this final reaction temperature, so obtain a maximum yield,typically about 70% of the stoichiometric amount of MnSO₄ converted toMnO₂. In this manner battery grade MnO₂ product can be obtained havingdensities comparable to or even higher than the density of electrolyticmanganese dioxide (EMD) which typically is at a level of about 25 to 28g/in³ (1.5 to 1.7 g/cm³) SAD (Scott Apparent Density). In general a bulkdensity of the MnO₂ product between about 15 and 32 g/in³ (0.9 and 2g/cm³) can be achieved by heating the aqueous solution of MnSO₄ and Na₂S₂ O₈ from an initial temperature between about 40° C. and 70° C. for aperiod during reaction at an average rate of less than about 7° C. perhour for at least 5 hours, typically between about 1° C. per hour and 7°C. per hour for at least 5 hours.

A sufficiently fast reaction rate to achieve an MnO₂ product bulkdensity of between about 8 to 15 g/cm³ (0.5 to 0.9 g/cm³) (ScottApparent Density) is obtained if the aqueous reaction mixture of MnSO₄and Na₂ S₂ O₈ is heated at about constant rate from room temperature sothat a final temperature of between 70° and 90° C. is achieved in aboutone to two hours. The reaction mixture may be left to stand for aboutone hour at this final temperature, to obtain a maximum yield, typicallyabout 70% of the stoichiometric amount of manganese in MnSO₄ convertedto MnO₂. In general a bulk density of the MnO₂ product between about 8g/in³ and 15 g/in³ (0.5 and 0.9 g/cm³) can be achieved by heating theaqueous solution of MnSO₄ and Na₂ S₂ O₈ from an initial temperaturebetween about 30° C. and 100° C. for a period during reaction at anaverage rate greater than 7° C. per hour for less than about 5 hours,typically between about 7° C. and 20° C. per hour for less than about 5hours.

It has been determined that the stoichiometric yield of MnO₂ can bedramatically increased to about 95% by slowly adding a suitable alkalinebase slowly to the reaction mixture. As the reaction proceeds the basereacts with the H₂ SO₄ as it forms, thereby improving the reactionkinetics and ultimate yield of MnO₂. A preferred base is Li₂ CO₃.Alternative bases can be employed to react with the H₂ SO₄ to producethe same increase in yield of MnO₂. Such compounds include Na₂ CO₃,LiOH, NaOH and MgO. For ultimate use of the MnO₂ product in lithiumcells it would be preferred to add compounds such as Li₂ CO₃ and LiOH tothe reaction mixture to increase yield. For ultimate use of the MnO₂product in alkaline cells it would be preferred to add Na₂ CO₃ or NaOHto the reaction mixture. If such compounds are added, they should beadded slowly to the reaction mixture to prevent the pH of the mixturefrom abruptly increasing to a pH greater than about 3.

The MnO₂ reaction product of the invention takes the form of descreteparticles having a spherical shape and gamma crystalline structure. Theparticle size of the MnO₂ reaction product can also be controlled byvarying the rate at which the reaction mixture is heated. If thereaction mixture is heated at a constant rate then the MnO₂ particlesize distribution will be uniform, that is, there will not be muchvariance in the diameter of individual MnO₂ particles. If the reactionmixture is slowly heated at constant rate, e.g., of between about 1° C.and 7° C. for at least 5 hours, the MnO₂ product will take the form ofrelatively large uniform spherical particles as above mentioned. If thereaction mixture is rapidly heated at a fast constant rate, e.g.,between about 7° C. per hour and 20° C. per hour for less than about 5hours, the MnO₂ product will tend to take the form of relatively smallspherical particles.

If the reaction mixture is initially heated at a slow constant rate andlater at a fast constant rate, the reaction product will contain adistribution of both large and small MnO₂ particles. While the overallshape of the individual MnO₂ particles produced is spherical, thesurface features of each particle is characterized by filament-like(e.g. hair-like) microscopic protrusions distributed uniformly over theentire particle surface. Such filament-like surface structure results inMnO₂ particles having a useful surface area which can be greater thanthat achieved with EMD, but yet the bulk density is comparable to thatof EMD. This is a benefit, particularly in lithium cells, because betterperformance and capacity is obtained when the useful surface area isincreased over that of EMD. The filament-like protrusions are visible inelectron photomicrographs taken at magnifications of between about 200and 2,000 times actual size.

The following examples illustrate the method of preparation of batterygrade MnO₂ by the the process of the invention. All parts are parts byweight unless specified otherwise.

EXAMPLE 1

High density gamma MnO₂ is prepared by the process of the invention asfollows:

120 g of MnSO₄ H₂ O is dissolved in 1800 ml of distilled water. Then,stoichiometric amount of Na₂ S₂ O₈ (169 g) is added to the clear pinkishsolution to form a reactant solution. While stirring, the solution isheated in about 2 hours from room temperature (20° C.) to 50° C. and ismaintained at a temperature of 50° C. overnight (about 18 hrs) whilecontinually stirring. This enhances the nucleation process. The reactionproceeds according to reaction (I) above referenced. The clear pinkishsolution slowly turns brown and then eventually turns a black color asmore MnO₂ is precipitated. After the 18 hour period the solution is thenheated from about 50° C. at a constant rate of about 25° C. per hour forabout 1 hour to a temperature of about 75° C. and is maintained at 75°C. for about 3 hours. The solution is then heated at constant rate ofabout 10° C. per hour for about 1 hour to a temperature of 85° C. andmaintained at 85° C. for 1 hour. The solution is again heated at aconstant rate 30° C. per hour for about 1/2 hour to a temperature ofabout 100° C. and maintained at 100° C. for about 11/2 hours at whichtime the run is ended. The pH of the solution at the end of the run isless than 0.5. The solution is then cooled to room temperature (20° C.)in about one hour. The solution is filtered and the solid MnO₂ iscontinually rinsed with distilled water until the filtrate stream has aneutral pH of about 7. The resulting black powder is dried at 100° C. todrive off surface water. The overall yield of MnO₂ is 41 g or 67% oftheoretical yield.

The resulting product is battery grade MnO₂ at least 95% of which isverified by x-ray diffraction to be of the gamma crystalline structure.(No other type MnO₂ crystalline structure was detected, the 95%threshold being the limit of resolution of the x-ray diffractionanalysis.) The MnO₂ product has a high bulk density of about 23 g/in³(1.4 g/cm³) SAD (Scott Apparent Density). An electron photomicrographrepresentative of this MnO₂ product is shown in FIGS. 1A and 1B. Theuniform spherical structure of the MnO₂ particles (e.g. particle 10) isshown in FIG. 1A taken at 199× magnification. The filament-like (e.g.hair-like) protrusions 15 emanating from the surface of each sphericalparticle are clearly visible in FIG. 1B, which shows an individualparticle at 2,030× magnification. By comparison the electronphotomicrographs of the commercial battery grade CMD (WSLi) particlesare shown in FIGS. 5A and 5B, which are taken at 202× and 2060×magnification, respectively. (The WSLi brand of CMD is available fromSedema, a division of Sadacem, S.A., Terte, Belgium.) It is clear fromFIGS. 5A and 5B that representative particles 70 do not exhibitfilament-like protrusions characteristic of the MnO₂ product of theinvention (FIGS. 1A and 1B).

EXAMPLE 2

Lower density gamma MnO₂ is prepared by the process of the invention asfollows:

The gamma MnO₂ product of the invention is made in a similar manner asdescribed in example 1, except that rate of heating is faster leading tosmaller size and less dense particles. Specifically, the same method ofpreparation and conditions as in example 1 are employed except thereactant solution is heated from 50° C. to 100° C. at rate of aboutabout 17° C. per hour for a period of less than 5 hours, namely about 3hours. FIGS. 2A and 2B are electron photomicrographs of the resultingMnO₂ product. The product sample represented in FIGS. 2A and 2B had abulk density of about 8.7 g/in³ (0.53 g/cm³) (Scott Apparent Density)and is at least 95% gamma MnO₂.

The filament-like (e.g. hair-like) surface protrusions 20 and 25 of theindividual MnO₂ particles may be seen in FIGS. 2A and 2B, respectively.The MnO₂ particles as described in this example may be used as cathodeactive material in electrochemical cells, particularly alkaline andlithium cells. If intended for use in lithium cells the gamma MnO₂ maybe heated at a temperature between about 300°-400° C., typically forabout 6 hours at 350° C. or 32 hours at 300° C. to convert the gammaMnO₂ to gamma-beta crystalline structure and to evaporate any residualmoisture entrapped within the MnO₂ particles.

EXAMPLE 3

The MnO₂ is produced in a manner similar to that described in Example 1except that Li₂ CO₃ is added to the reaction mixture in order toincrease the yield of MnO₂. 583 g of MnSO₄ H₂ O is first dissolved in 8liter of distilled water in a 12 liter round bottom flask. Thenstoichiometric amount of Na₂ S₂ O₈ (822 g) is added to the slightlypinkish solution. The solution is heated at a constant slow rate for 6hours from room temperature (20° C.) to 55° C. Then 23 g of Li₂ CO₃ isthen slowly added and the solution is maintained at a temperature ofabout 55° C. for 18 hours while continually mixing. An additional 69 gof Li₂ CO₃ is added after the 18 hour period and the solution is heatedat a constant rate of about 6° C. per hour for about 2.5 hours up to atemperature of 70° C. Another 36 g of Li₂ CO₃ is then added and thesolution is heated at a constant rate of about 5° C. per hour for 2hours up to a temperature of about 80° C. The solution is then heated ata reduced constant rate of about 3.3° C. per hour for 3 more hours up toa temperature of 90° C. The solution temperature is held for about 18hours and then cooled in about 1 hour to room temperature (20° C.). TheMnO₂ product is recovered and dried in the manner described inexample 1. The yield of MnO₂ is 270 g or 90% of the theoretical yield.At least 95% of the MnO₂ product is verified by x-ray diffraction to begamma MnO₂. The bulk density of the MnO₂ product is measured as 20 g/in³(1.2 g/cm³) (Scott Apparent Density). This MnO₂ product can be heattreated as in Example 1 whereupon it becomes particularly suitable foruse as a cathode active material in lithium cells.

EXAMPLE 4

This example demonstrates the use of EMD particles as a substrate forthe precipitation of MnO₂ in accordance with the invention.

120 g of MnSO₄ H₂ O is dissolved in 1.6 liter of distilled water in a 2liter beaker by stirring. 120 g of Na₂ S₂ O₈ and 20 g of EMD (fromKerr-McGee) are then added to the slightly pinkish clear solution.

The heating regimen is as follows. The whole mixture is first heatedfrom room temperature (20° C.) to 55° C. in about 2 hours and held atthis temperature for 18 hours while continually mixing. The wholemixture is then heated slowly at constant rate for about 5.5 hours to atemperature of 75° C. Then the whole mixture is heated for another hourat constant rate to a temperature of 100° C. Thereupon the mixture iscooled to room temperature (20° C.) in about 1 hour.

The hybrid MnO₂ is rinsed with distilled water until neutral. Then it isfiltered and dried at 100° C. to remove surface water. The total yieldof hybrid MnO₂ product is 60 g and its bulk density is 24 g/in³ (1.5g/cm³) (Scott Apparent Density). The hybrid MnO₂ product contains about67 wt % of the deposited gamma MnO₂ and 33 wt % EMD.

The MnO₂ product consists of gamma MnO₂ deposited uniformly over thesurface of the individual EMD particles to form a hybrid MnO₂ product.Each particle of the hybrid MnO₂ product retains the overall irregularshape of the EMD particle, but exhibits a surface formed of uniformlydistributed filament-like protrusions characteristic of the gamma MnO₂made in accordance with the process of the invention. Representativeelectron photomicrographs of the hybrid MnO₂ particles are shown inFIGS. 4A and 4B. By way of comparison FIGS. 3A and 3B are electronphotomicrographs of the EMD particles. These figures clearly show theirregular shape and smooth surface of each EMD particle. FIG. 4A showsthe overall shape of each hybrid particle, e.g., particle 60 (at amagnification of 450 times actual), as resembling the shape of the EMDparticles, e.g. particle 50 (FIG. 3A). However, as may be seen from FIG.4B, the surface features of each hybrid MnO₂ particle exhibitfilament-like protrusions, e.g. protrusions 65, emanating from anduniformly covering the surface of each hybrid particle. This is theresult of the deposition of the gamma MnO₂ of the present process ontothe EMD particles. An advantage of this hybrid is that it has highersurface area than EMD, but yet also has high bulk density. It is alsocheaper to manufacture than an equivalent weight of gamma MnO₂ producedby the process of the invention. The hybrid MnO₂ so produced can be usedas cathode active material in electrochemical cells. If heat treatedbefore application, e.g. as in Example 1, it can be employed as cathodeactive material in lithium cells.

EXAMPLE 5

This example demonstrates the preparation of high density gamma MnO₂specifically for use as cathode active material in alkaline cells.

583 g of MnSO₄ H₂ O are dissolved in 8000 ml of distilled watercontained in a 12 liter round bottom flask. Then, stoichiometric amountof Na₂ S₂ O₈ (822 g) is added to the clear pinkish solution. Thesolution is heated from room temperature (20° C.) to 50° C. in about 2hours. The solution is then slowly heated from 50° C. to 65° C. over aperiod of eight hours and maintained at a temperature of 65° C. for 18hours while continually stirring. The reaction proceeds according toreaction (I) above referenced. The clear pinkish solution slowly turnsto a brown and then eventually black color as more MnO₂ is deposited.Following the 18 hour period the solution is then finally heated slowlyat about a constant rate from 65° C. to 80° C. over a period of eighthours. The solution is cooled to room temperature (20° C.) in about 1hour. The gamma MnO₂ product is recovered by filtering the finalsolution and continually rinsing with distilled water until the filtratehas a neutral pH of about 7. The resulting black powder is dried as inthe preceeding examples to drive off surface water. The resultingproduct is battery grade MnO₂ which is verified by x-ray diffraction tobe of the gamma crystalline structure. The MnO₂ product has a high bulkdensity of about 28 g/in³ (1.7 g/cm³) SAD (Scott Apparent Density). (Forusage in an alkaline cell, the MnO₂ product of the invention preferablyshould exhibit a high SAD, preferably of at least 25 g/in³ (1.5 g/cm³)which in turn has been found to result in a high load voltage andcapacity.)

PERFORMANCE TESTS EXAMPLE 6

The MnO₂ product of the invention (P-CMD) is evaluated for itselectrochemical performance in an AA cell. The performance of the MnO₂product (P-CMD) as cathode active material in an alkaline AA cell isshown in FIGS. 6A. The performance of the MnO₂ product (P-CMD) as shownin this figure is compared to conventional EMD cathode active material(from Kerr-McGee Corp.) for the same type cell. It is clear from thisfigure that the MnO₂ product (P-CMD) exhibits a slightly higher runningvoltage and a greater capacity (amp-hrs) than obtainable for the samecell using EMD as cathode material. The MnO₂ product (P-CMD) is believedto be the first CMD that exhibits better performance in alkaline cellsthan EMD.

EXAMPLE 7

The MnO₂ product of the invention is evaluated for its performance in aflooded alkaline cell. This cell utilizes conventional zinc anode andKOH electrolyte and paper separator as employed in commercial Duracellalkaline cells. The flooded cell is in the shape of a disk of samediameter as that of a Duracell AA cell. The flooded cell is cathodelimited, thus excess electrolyte (1.5 g) and excess zinc (5.6 g) areused in order to evaluate the intrinsic performance of the MnO₂ product(0.17 g) as cathode active material. The flooded cell is fabricated byfirst pouring a mixture of MnO₂ powder, graphite and KOH (60 wt % MnO₂,34.5 wt % graphite and 5.5 wt % KOH) into the bottom of an empty AA sizenickel coated stainless steel can which is open at one end and closed atthe other. The MnO₂ powder is then compacted into a disk-like shape. Apaper separator is then placed on top of the MnO₂ disk. The separator isthen filled with the KOH electrolyte and the remaining volume of the canthen filled with a zinc slurry. The open end of the can is covered witha stainless steep cap. The cap is in electrical contact with the zincslurry through a nail penetrating from the cap into the slurry.

Two flooded cells are made as above described, but with one containingthe MnO₂ product of the invention (P-CMD) as cathode material and theother containing conventional battery grade EMD (from Kerr-McGee Co.) ascathode material. The performance of the two cells are compared at acurrent drain rate of 0.3 milli-amp/cm² and the results shown in FIG.6B. It may be seen from the voltage profiles reported in FIG. 6B thatthe performance of the flooded alkaline cell utilizing the MnO₂ product(P-CMD) is superior to that employing the EMD.

EXAMPLE 8

The gamma MnO₂ product obtained by the process described in example 3 isheated at about 350° C. for about six hours to convert the gamma MnO₂ toa gamma-beta phase. A coin shaped cell is fabricated utilizing a cathodeactive material prepared by mixing MnO₂, graphite andpolytetrafluouroethylene binder in a weight ratio of 6:3:1. The cathodemixture is compacted by press molding it onto a stainless steel mesh andspot welding it onto a steel case which forms the positive electrode.The positive electrode containing the cathode material is immersed in aconventional lithium salt electrolyte composed of lithiumhexafluorophosphate (LiPF₆) dissolved in propylene carbonate anddimethoxyethane organic solvents. Other conventional lithium saltelectrolytes such as lithium perchlorate and organic solvents such aspropylene carbonate, ethylene carbonate, dimethoxyethane and mixturesthereof can also be used. Excess amount of lithium is employed for thenegative electrode. The negative electrode is formed by press molding alithium foil onto a stainless steel mesh which in turn is spot welded toa steel case. A separator composed of a non-woven cloth is applied overthe lithium foil. The positive electrode is assembled over the negativeelectrode with the separator therebetween. The assembly is performed inan argon filled dry chamber. The entire assembly is filled with theliquid electrolyte and then sealed by crimping the edge of the cell.

Two lithium coin-shaped cells made in the above manner are dischargeddown to 1.2 volts with current drain rates of 0.17 and 1 milliamp/cm²,respectively. The resulting voltage profiles for these cells utilizingthe MnO₂ cathode active material of the invention (P-CMD) are shown inFIGS. 7A and 7B for drain rates at 0.17 and 1 milliamp/cm²,respectively. Each figure also shows comparative voltage profilesobtained for a like cell at same current drain rates, but instead usingconventional EMD cathode active material (from Kerr-McGee Corp.) whichis heat treated and press molded for use in the lithium cell. As may beseen from the figures the MnO₂ cathode active material of the invention(P-CMD) exhibits a greater capacity (milliamp-hr/g) than the EMD. Thecapacity improvement of the MnO₂ product of the invention over that ofEMD for the current drain rates of 0.17 and 1 milliamp/cm² are about 20%and 28%, respectively. The MnO₂ product of the invention, thus, showsperformance improvement over EMD in lithium cells, particularly at thehigher current rates.

EXAMPLE 9

The same tests are performed as in example 8 using the coin-shapedlithium cells assembled, as above described, except that the performanceof the MnO₂ product of the invention (P-CMD) is compared against that ofCMD. The CMD chosen is that commercially available for specific use inlithium cells, namely, that sold under the trade designation WSLichemical manganese dioxide (CMD) for Sedema, division of Sadacem, S.A.,Tertre, Belgium.

Two coin-shaped lithium cells are prepared as in example 8 but with onecell containing Sedema WSLi chemical manganese dioxide and the othercontaining the MnO₂ product of the invention (P-CMD) as the cathodeactive material. The voltage profiles for these two cells are given atcurrent drain rates of 0.17 and 1.0 milliamp/cm² as illustrated in FIGS.8A and 8B, respectively. As may be seen from these figures, the MnO₂product of the invention (P-CMD) has significantly greater capacity(milliamp/g) than the Sedema CMD (WSLi) at the current drain ratestested.

Although the present invention has been described with reference tospecific embodiments, it should be recognized that variations arepossible within the scope of the invention. Therefore, the invention isnot intended to be limited to specific embodiments, but rather isdefined by the claims and equivalents thereof.

What is claimed is:
 1. An electrochemical cell having MnO₂ cathodeactive material in said cell, wherein the MnO₂ material comprisesparticles the surfaces of which comprise gamma MnO₂ having filamentaryprotrusions radiating outwardly from the particles, said filamentaryprotrusions being visible at a magnification of between about 200 and2000 times actual size and not visible at a magnification of less thanabout 200 times actual size.
 2. The electrochemical cell of claim 1,wherein said electrochemical cell has an anode comprising materialselected from the group consisting of lithium and zinc.
 3. Theelectrochemical cell of claim 1 wherein said MnO₂ material comprises atleast 95% gamma MnO₂.
 4. The electrochemical cell of claim 1 wherein thefilamentary protrusions are substantially uniformly distributed over thesurface of said particles and the filamentary protrusions have a lengthto width ratio between about 2:1 and 20:1.
 5. An electrode for anelectrochemical cell comprising MnO₂ particles comprising at least 95%gamma MnO₂ having filamentary protrusions of MnO₂ radiating outwardlyfrom the surface of said particles, said filamentary protrusions beingvisible at a magnification of between about 200 and 2000 times actualsize and not visible at a magnification of less than about 200 timesactual size.
 6. The electrochemical cell of claim 2 wherein the anodecomprises lithium and said cell further comprises a non-aqueouselectrolyte.
 7. An electrochemical cell having MnO₂ cathode activematerial in said cell, wherein the MnO₂ material comprises at least 95%gamma MnO₂ particles the surfaces of which have filamentary protrusionsradiating outwardly therefrom, said filamentary protrusions beingvisible at a magnification between about 200 and 2000 times actual sizeand not visible at a magnification of less than about 200 times actualsize.
 8. An electrochemical cell having anode active material, a cathodeactive material and electrolyte in said cell, said cathode activematerial comprising a hybrid MnO₂ material comprising gamma MnO₂ on thesurface of particles of electrolytic manganese dioxide (EMD), saidhybrid MnO₂ material having filamentary protrusions of MnO₂ radiatingoutwardly therefrom, said filamentary protrusions being visible at amagnification of between about 200 and 2000 times actual size and notvisible at a magnification of less than about 200 times actual size. 9.The electrochemical cell of claim 8 wherein said electrochemical cellhas an anode active material comprising lithium.
 10. The electrochemicalcell of claim 8 wherein said electrochemical cell has an anode activematerial comprising zinc.
 11. The electrochemical cell of claim 8wherein said electrochemical cell is an alkaline cell.
 12. Anelectrochemical cell having anode active material, a cathode activematerial and electrolyte in said cell, said cathode active materialcomprising a hybrid MnO₂ material comprising gamma MnO₂ on the surfaceof particles comprising carbonaceous material, said hybrid MnO₂ materialhaving filamentary protrusions of MnO₂ radiating outwardly therefrom,said filamentary protrusions being visible at a magnification of betweenabout 200 and 2000 times actual size and not visible at a magnificationof less than about 200 times actual size.
 13. The electrochemical cellof claim 12 wherein the anode active material comprises lithium.
 14. Theelectrochemical cell of claim 12 wherein the anode active materialcomprises zinc.
 15. The electrochemical cell of claim 12 wherein saidelectrochemical cell is an alkaline cell.